Hi, Sakon!  I don't know if it will answer your 
question or not, but in my experience (with one 
oversized cel with bends) that yeah, putting it 
in the cel book did flatten it out nicely.  ;)  I 
also had the book simply lying on a shelf which 
in itself I have heard is bad, but no damage was 
done to the other cels and I think the weight 
helped.

Frankie :)